How often do I need to change baby's bedsheets?

I change my baby's bedsheet once per week and vacuum the mattress once per month to remove dust mites. For me, I believe if by just washing the bedsheet it is not enough, thus I invested in dust mite vacuum. My brand is Delphine ($3k) and nowadays there are many other brands which has this function and much cheaper, e.g. Philips Mite Vacuum Cleaner which costs less than $300. http://www.philips.com.sg/c-p/FC6232_02/mite-cleaning-vacuum-cleaner If your baby has allergies, it is recommended to change at least 2 times per week.